Organopolysiloxanes having an atomic ratio of oxygen to silicon of 1.5 are generally called polysilsesquioxanes. These resins are utilized as abrasion-resistant coatings particularly on plastic lenses and glazing material. Polysilsesquioxanes have a ladder-like structure represented by the formula ##STR1## wherein R is the same or different and is lower alkyl, halolower alkyl, phenyl and substituted phenyl, with at least one of the groups represented by R being lower alkyl, and n is an integer.
Coating compositions containing such resins are resistant to high temperatures and are very hard. The hardness is due to a high degree of cross-linking in the resin. Although it is necessary that the resin coating be hard, the high degree of cross-linking also makes the coating brittle and subject to cracking upon prolonged exposure to high temperatures.
The brittleness of polyalkylsilsesquioxanes as represented by the above formula wherein R is lower alkyl has heretofore been reduced by replacing a portion of the alkyl groups with phenyl or certain halolower alkyl groups such as 3,3,3-trifluoropropyl groups. The presence of these groups in the structure effectively reduces cross-linking by physical hinderance. The disadvantage to the use of phenyl substitution to toughen the resins is that they absorb ultraviolet radiation which will cause the resin to degrade. This problem is particularly acute when it is desired to utilize the coatings on space hardware, such as satellites, which must endure intense ultraviolet radiation and, in a low earth orbit, atomic oxygen. Although the 3,3,3-trifluoropropyl groups do not absorb ultraviolet radiation, they lower the high temperature properties of the resin to a degree which can be disadvantageous in certain applications.
Another problem inherent in presently available polyalkylsilsesquioxane preparations is that the catalysts conventionally utilized in their preparation are organic acids, such as acetic acid. These acid catalysts are ionic and therefore, corrosive, a disadvantage when the coating is to be utilized on metal. It is also necessary to package the catalysts separately from the resin monomer and combine them just prior to use as is done with epoxy resin preparations. These catalysts are, for the most part, also volatile which means that there is the potential for the catalyst to be lost before the resin is completely formed.
In accordance with this invention, polyalkylsilsesquioxane coating compositions are provided which suffer none of these disadvantages.